As one example of a flow meter device for measuring a flow (flow rate) of a fluid by utilizing an ultrasonic wave, there is known a flow meter device which employs an inverse transit time difference method. In the inverse transit time difference method, ultrasonic transducers (transmitters/receivers) are provided at an upstream side and a downstream side, respectively, of a fluid passage (measurement fluid passage) which is a measurement target, and alternately transmit and receive an ultrasonic wave pulse. This makes it possible to measure the flow velocity of the fluid by utilizing the transit time (propagation time) in a forward direction and the transit time in a reverse direction. Therefore, the flow of the fluid can be measured by using the flow velocity and the cross-sectional area of the measurement fluid passage.
As a specific example of the flow meter device which employs the inverse transit time difference method, there is typically an estimation gas meter. The estimation gas meter is typically configured to intermittently measure the flow of the gas within a pipe which is the measurement fluid passage by sampling, calculate an average value of the measurement values, and integrate the average values, thus obtaining a gas usage amount (integrated flow value). This sampling measurement is fundamentally performed once in every preset sampling cycle. That is, the sampling cycle revers to a cycle set as a minimum time unit at which the flow measurement is performed.
In the estimation gas meter having the above configuration, it is not necessary to include a mechanical activation section for the flow measurement, as compared to a conventional actual capacity (actual measurement) gas meter (e.g., membrane gas meter). Because of this, in the estimation gas meter, the number of components can be reduced, and therefore reduction of a size and cost can be attained.
By the way, the gas is a compressive fluid, differently from a liquid such as water. Therefore, a pulsation is more likely to occur in a gas flow while the gas is passing through a device which compresses the gas, such as a gas engine heat pump (GHP). It is known that in the estimation gas meter, an error occurs in the gas flow measurement if this pulsation overlaps with the sampling cycle.
Especially, in the case of occurrence of cyclic pulsation, if the pulsation cycle and the sampling cycle coincide with each other or get close to each other, or the pulsation cycle is an integer multiple of the sampling cycle, there may be a possibility that only peak values or bottom values of the pulsation are measured as gas flow values. In such cases, an error of the gas flow values becomes relatively large. If the flow values containing the error are integrated, the gas usage amount which is the integrated value would significantly deviate from an actual usage amount.
Under the above stated circumstances, there have been conventionally proposed various techniques for obtaining accurate gas flow values even when a pulsation occurs in the gas flow. For example, Patent Literature 1 discloses that a flow measuring method and a flow meter device which change the phase of start of a measurement period according to a predetermined rule. Specifically, a time length Td is set in each measurement period T, and the cycle or phase of timing of start of each of the plurality of measurement periods T is changed according to the predetermined rule so that this cycle or phase is made different from the cycle or phase of the pulsation actually measured.
Patent Literature 1: Japanese-Laid Open Patent Application Publication No. 2001